The present invention relates to a magnetic tunnel junction device, a magnetic memory adopting the magnetic tunnel junction device, a magnetic memory cell and an access method of the magnetic memory cell.
Magnetic tunnel junction (MTJ) devices are known to output a signal of a higher level as compared to conventional anisotropic magnetoresistive (AMR) effect devices and giant magnetoresistive (GMR) effect devices. For this beneficial features of the magnetic tunnel junction (MTJ) devices, their applications to reproducing head for hard disk drives (HDDs) or magnetic memories have been considered.
Especially, in the magnetic memories, the MTJ devices are solid memory of no active section like the semiconductor memories. However, the MTJ devices are advantageous over the semiconductor memories for their beneficial features: a) information stored therein will not be lost in an event of power shut off, b) rewriting of information is permitted unlimited number of times, c) even with an applied radioactive ray, the information stored therein would not be lost.
An example structure of conventional MTJ device is shown in FIG. 11. Such structure is, for example, disclosed by Japanese Unexamined Patent Publication No. 106514/1997 (Tokukaihei 9-106514, published on Apr. 22, 1997).
As shown in FIG. 11, an MTJ device 104 includes an antiferromagnetic layer 141, a ferromagnetic layer 142, an insulating layer 143, and a ferromagnetic layer 144 which are laminated in this order. Both the ferromagnetic layer 142 and the ferromagnetic layer 144 have in-plane magnetizations which show such effective uniaxial magnetic anisotropy that the respective magnetizations are parallel to or antiparallel to one another. The magnetization of the ferromagnetic layer 142 is fixed in substantially one direction by an exchange coupling with the antiferromagnetic layer 141, and thus information is recorded thereon in a magnetization direction of the ferromagnetic layer 144.
For the antiferromagnetic layer 141, FeMn, NiMn, PtMn or IrMn alloy may be adopted. For the ferromagnetic layer 142 and the ferromagnetic layer 144, Fe, Co, Ni, or alloys thereof may be adopted. For the insulating layer 143, various kinds of oxides, nitrides, etc., may be used. However, it is known that the highest magnetoresistance (MR) can be obtained when adopting an Al2O3 film for the insulating layer 143.
Another MTJ device has been proposed of a structure without the antiferromagnetic layer 141, which utilizes a difference in coercive force between the ferromagnetic layer 142 and the ferromagnetic layer 144.
An operation mechanism of a magnetic memory application of the described MTJ device 104 of FIG. 11 is shown in FIGS. 12(a) and 12(b).
As described, both the ferromagnetic layer 142 and the ferromagnetic layer 144 have in-plane magnetization and show such effective uniaxial magnetic anisotropy that the respective magnetization are parallel to or antiparallel to one another.
The magnetization of the ferromagnetic layer 142 is fixed in substantially one direction by an exchange coupling with the antiferromagnetic layer 141, and a recording is performed based on the magnetization direction of the ferromagnetic layer 144.
Reading out operation is performed by detecting a resistance of the MTJ device 104 which differs depending on whether the magnetization of the memory layer of the ferromagnetic layer 144 and the magnetization of the ferromagnetic layer 142 are parallel to or antiparallel to each other.
FIG. 13 shows a schematic structure of a randomly accessible magnetic memory adopting the MTJ device of FIG. 11. The magnetic memory includes a transistor 151 for selecting an MTJ device 152 when reading out. The respective information represented by xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d are recorded based on the magnetization direction of the ferromagnetic layer 144 of the MTJ device 104 shown in FIG. 11. Then, information are read out utilizing the feature that the resistance value is low when the magnetization of the ferromagnetic layer 142 and the magnetization of the ferromagnetic layer 144 are parallel to one another, while the resistance value is high when the respective magnetization are antiparallel to one another.
On the other hand, recording is performed by inverting the magnetization direction of the ferromagnetic layer 144 by a synthetic magnetic field formed by a bit line 153 and a word line 154. A reference numeral 155 in FIG. 13 indicates a plate line.
In the described MTJ device 104, however, magnetic poles generate at both end portions due to the in-plane magnetization of the ferromagnetic layer 142 and the ferromagnetic layer 144. In order to meet a demand for high density magnetic memory, a miniaturization of the MTJ device 104 is necessary. However, the miniaturization of the MTJ device 104 results in a greater effect of the demagnetizing field caused by the magnetic poles generated at both end portions.
The ferromagnetic layer 142 which is exchange coupled with the antiferromagnetic layer 141 is not affected much by the described demagnetizing field. Further, as disclosed in U.S. Pat. No. 5,841,692, the magnetic poles generated at both end portions can be substantially eliminated.
However, the same cannot be applied to a memory layer of the ferromagnetic layer 144, and as the pattern is miniaturized, the magnetization becomes more unstable due to the magnetic poles generated at both end portions, thereby making it difficult to maintain the initial recorded state.
In order to counteract the foregoing problem, attempts have been made to suppress the effects of the magnetic poles at both end portions by arranging the memory layer of the ferromagnetic layer 144 so as to have a closed magnetic circuit structure. In this state, by arranging such that both the bit line and the word line are formed in the closed magnetic circuit, the magnetization direction of the ferromagnetic layer 144 can be inverted effectively when recording. However, since the bit line and the word line are formed in the same direction within the MTJ device, it is difficult to adopt the simple orthogonal array as shown in FIG. 13. The foregoing closed magnetic circuit structure is disclosed in, for example, Japanese Unexamined Patent Publication No. 302456/1998 (Tokukaihei 10-302456, published on Nov. 13, 1998), however, an optimal access method of the magnetic memory cell is not disclosed.
When applying the MTJ device to the magnetic head or the magnetic memory, it is important to attain a high resistance change ratio. However, an improvement in resistance change ratio by an optimal selection of a material is limited, and thus a method of improving resistance change ratio by modifying the film structure has been considered. For example, Japanese Unexamined Patent Publication No. 163436/1999 (Tokukaihei 11-163436, published on Jun. 18, 1999) realizes a high resistance change ratio by laminating a plurality of MTJ devices.
The structure of the MTJ device of Japanese Unexamined Patent Publication No. 163436/1999 is shown in FIG. 14. As shown in FIG. 14, the MTJ device includes a first ferromagnetic layer 161, a first insulating layer 162, a second magnetic layer 163, a second insulating layer 164 and a third magnetic layer 165 which are laminated in this order. The described MTJ device is mainly designed for the magnetic head; however, it can be also applied to magnetic memory.
In the case of applying the MTJ device of the structure shown in FIG. 14, the arrangement wherein the ferromagnetic layer 161, the ferromagnetic layer 163 and the ferromagnetic layer 165 all show in-plane magnetization which show such effective uniaxial magnetic anisotropy that the respective magnetization are parallel to or antiparallel to one another.
The respective magnetization of the ferromagnetic layer 161 and the ferromagnetic layer 165 are fixed in substantially one direction by the exchange coupling with the antiferromagnetic layer, and information is recorded based on the magnetization of the ferromagnetic layer 163. On the other hand, reading out of information is performed by detecting the resistance of the MTJ device which differs depending on whether the magnetization of the memory layer of the ferromagnetic layer 163 is parallel or antiparallel to other ferromagnetic layers 161 and 165. According to the structure shown in FIG. 14, the magnetic tunnel junction sections are connected in series, and thus, can output a signal of a level twice as high as the conventional MTJ device with a single magnetic tunnel junction section.
Writing is performed by changing a magnetization direction of the ferromagnetic layer 163 by utilizing a magnetic field generated by a current line disposed in a vicinity of the MTJ device.
In the described MTJ device also, the ferromagnetic layer 161, the ferromagnetic layer 163 and the ferromagnetic layer 165 have in-plane magnetization, and thus magnetic poles generate at both end portions. In order to realize high density magnetic memory, it is required to miniaturize the MTJ device. However, the miniaturization of the MTJ device results in a greater effect of the demagnetizing field caused by the magnetic poles generated at both end portions.
In the case where the ferromagnetic layers 161 and 165 are exchange coupled with the antiferromagnetic layer, the demagnetizing field is not affected much. By constituting the ferromagnetic layers 161 and 165 with two ferromagnetic layers which are exchange coupled, magnetic poles generated at both end portions are substantially zero.
However, the same cannot be applied to a memory layer of the ferromagnetic layer 163, and as the pattern is miniaturized, the magnetization becomes more unstable due to the magnetic poles generated at both end portions, thereby making it difficult to maintain the initial recorded state.
It is an object of the present invention to provide a magnetic tunnel junction device which can maintain a magnetization recorded in a memory layer under stable condition even for a miniaturized pattern, and a magnetic memory adopting the same.
It is another object of the present invention to provide an access method of a magnetic memory which permits a closed magnetic circuit structure to be introduced in a ferromagnetic layer of a memory layer without reducing a cell density of a magnetic memory.
In order to achieve the above object, the magnetic tunnel junction device of the present invention including at least a first magnetic layer, an insulating layer and a second magnetic layer which are laminated in this order, is characterized by further including:
a third magnetic layer formed on an opposite side of an insulating layer forming side of the first or second magnetic layer via a metal layer with a spacing at a central portion,
wherein the first or second magnetic layer and the third magnetic layer form a closed magnetic circuit.
According to the forgoing arrangement, the magnetization of the third magnetic layer serving as the closed-magnetic circuit layer and the first or second magnetic layer form a closed loop; and thus the generation of the magnetic poles at both end portions can be avoided. As described, according to the MTJ device of the described structure, effects of the magnetic poles generated at both end portions can be avoided. Thus, a stable magnetization state as recorded can be maintained even for a miniaturized pattern. Additionally, since the memory layer of the ferromagnetic layer has a closed magnetic circuit structure, stable magnetization state as recorded can be ensured against an external leakage magnetic field.
In order to achieve the above object, a magnetic memory of the present invention is characterized by adopting a magnetic tunnel junction device including at least a first magnetic layer, an insulating layer and a second magnetic layer which are laminated in this order, and a third magnetic layer formed on an opposite side of an insulating layer forming side of the first or second magnetic layer via a metal layer with a spacing at a central portion, wherein the first or second magnetic layer and the third magnetic layer form a closed magnetic circuit.
According to the forgoing arrangement, the magnetization of the third magnetic layer serving as the closed magnetic circuit layer and the first or second magnetic layer form a closed loop; and thus the generation of the magnetic poles at both end portions can be avoided. As described, according to the MTJ device of the described structure, effects of the magnetic poles generated at both end portions can the eliminated. Thus, a stable magnetization state as recorded can be maintained for a miniaturized pattern. Additionally, since the memory layer of the ferromagnetic layer has a closed magnetic circuit structure, a stable magnetization state as recorded can be ensured against an external leakage magnetic field. Therefore, a power consumption of the magnetic memory adopting the MTJ device can be reduced.
In order to achieve the above object, another magnetic tunnel junction device of the present invention including a first magnetic layer, a first insulating layer, a second magnetic layer, a second insulating layer and a third magnetic layer which are laminated, is characterized by further including:
a fourth magnetic layer formed on either a first insulating layer forming side or a second insulating layer forming side of the second magnetic layer via a metal layer with a spacing at a central portion, wherein:
the second magnetic layer and the fourth magnetic layer form a closed magnetic circuit.
According to the described arrangement, a high resistance change ratio can be obtained, and the effects of the magnetic poles generated at both end portions can be reduced. Thus, a stable magnetization state as recorded can be maintained even for a miniaturized pattern. Additionally, since the memory layer of the ferromagnetic layer has a closed magnetic circuit structure, stable magnetization state as recorded can be ensured against an external leakage magnetic field.
In order to achieve the above object, another magnetic memory of the present invention is characterized by including: a first magnetic layer, a first insulating layer, a second magnetic layer, a second insulating layer and a third magnetic layer which are laminated, and further include a fourth magnetic layer formed either a first insulating layer forming side or a second insulating layer forming side of the second magnetic layer via a metal layer with a spacing formed at a central portion, wherein the second magnetic layer and the fourth magnetic layer form a closed magnetic circuit.
According to the described arrangement, a high resistance change ratio can be obtained, and the effects of the magnetic poles generated at both end portions can be reduced. Thus, a stable magnetization state as recorded can be maintained for a miniaturized pattern. As a result, a magnetic memory of an improved integration can be realized, and a power consumption of the magnetic memory can be reduced.
In order to achieve the above object, an access method of a magnetic memory cell of the present invention is characterized by including the steps of:
forming a closed magnetic circuit on a magnetic layer for storing a memory of a magnetic memory cell;
placing a first current line (bit line) in a closed magnetic circuit composed of the magnetic layer and the closed magnetic circuit layer, and a second current line (word line) outside the closed magnetic circuit;
changing the magnetization direction of the magnetic layer by applying such current to the first current line that the magnetization of the closed magnetic circuit layer is inverted but the magnetization of the magnetic layer is not inverted, and applying such current to the second current line that the magnetization of the magnetic layer is not inverted by the second current line alone, but the magnetization of the magnetic layer is inverted by a synthetic magnetic field with the first current line.
According to the described access method of the magnetic memory cell, by controlling the magnetic characteristics of the ferromagnetic layer and the closed magnetic circuit layer and the level of current flowing through the first current line (bit line) and the second current line (word line), access to only one magnetic memory cell is permitted, and a required recording current can be reduced. Further, the effects of magnetic poles at both end portions on the magnetic memory cell can be suppressed. As a result, a stable magnetization state can be ensured even for a miniaturized pattern, and a highly integrated magnetic memory can be realized. Additionally, by adopting the closed magnetic circuit layer structure for the memory layer, a stable magnetization state as recorded can be ensured against an external leakage magnetic field. Further, a power consumption of the magnetic memory can be reduced.
Further, the first current line is formed within the closed magnetic circuit layer structure formed by the magnetic layer and the closed magnetic circuit layer, the magnetization of the closed magnetic circuit layer can be inverted at sufficiently low current, thereby effectively applying the magnetic layer.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.